Immonen's a great artist - but I've never liked his Ultimate stuff. I don't know it looked kinda bland and lifeless, even his work on UXM.
Bagley, for all his flaws, churned out issue after issue and what he lacked in refinement he made up for in dynamism. And he got better as the run went on. Bagley on USM was like Howard Porter on Morrison's JLA.
Bagley is my favourite artist in comics and USM is a big reason why, so I'm gonna have to go with him.
That being said I never thought Immonen did a bad job on the title, the only thing that seemed off about his artwork to me was that he seemed to have trouble doing hands, he wouldn't do them too badly, but they weren't as good as his artwork on everything else, so it just made it stick out.